Rotary magnetron sputtering is well known in the art following McKelvey, AS EVIDENCED BY U.S. Pat. No. 4,446,877. FIGS. 1A, 1B, 2A, and 2B show a conventional, prior art rotary magnetron with a sputter racetrack 2 proximal to the outer surface of the target tube cylinder 1. the target cylinder 1 is supported on a backing tube 14. The sputter racetrack 2 generates a plasma region proximal to the target material 10 to induce removal of target material 10 and onto a deposition substrate. As is known, the sputter target material 10 is formed into a target cylinder 1 and the cylinder 1 is rotated on spindle 6 while an internal magnet bar 20 is held stationary within the cylinder 2, as shown in FIG. 2A. The result is a sputter magnetron racetrack 2 of deposition plasma appears on the rotating tube during operation, as best seen in FIG. 1A that is a magnified longitudinal cross sectional view of region IB of FIG. 1A. The racetrack 2 has linear portions 4 and turnaround portions 3. The approximate registry is shown between an interior edge 5 of turnaround portion 3 and the point of maximal target wear 7 and is a result of the differences in plasma density and magnetic electron confinement to which the target material is exposed proximal to turnaround portion 3 relative to linear portions 4. As will be detailed subsequently, the magnetic field generated by the magnet bar is also important in establishing the point of maximal erosion 7. It is appreciated that the opposing end of the target cylinder 1 has a mirror image erosion zone to that shown in the accompanying figures associated with the other end of the racetrack 2. This mirror image zone is not shown for visual clarity yet is otherwise identical in formation and shape to that depicted.
As is known, while considerably better than planar magnetron sputtering, target utilization is only approximately 60% for a typical rotary magnetron. The reason for this is premature target wear of the cylinder 1 in the vicinity of the turnaround 3 of the sputter racetrack 2. This is shown with greater clarity in the FIG. 1B cross-section view. The target cylinder 1 before usage has a surface 15 that defined an initial target thickness. As target tube material 10 is sputtered off the target cylinder 1, the cylinder section 13 proximal to the turnaround portion 3 of the racetrack 2 wears faster than the cylinder section 12 proximal to a straightway portion of the racetrack 2. The operational lifetime of a target tube cylinder 1 is exhausted when the target material is worn through almost to backing tube 14. Sputtering of the backing tube 14 onto the substrate contaminates the deposited film. Due to the faster wear, this occurs at a cylinder section 13 first, leaving considerable target material 10 unusable along straight away section 12 underlying straight portion 4. A conventional solution to this detrimental turnaround wear pattern is the usage of a target with added thickness in the end regions and underlying the turnaround portions 3. Such targets are commonly referred to as “dog-boned”. While the thicker dog-boned region improves target utilization, this comes are the cost of more complicated target formation and a larger overall target diameter to the target. The increased separation distance between the target surface and a substrate for coating increases magnetron energy consumption and overall efficiency.
FIG. 2A shows a more detailed longitudinal cross-sectional view proximal to the turnaround portion 3 of a conventional, prior art rotary magnetron racetrack 2 along line of FIG. 1A. The backing tube 14 is shown in partial cutaway for visual clarity. In this cross-sectional view the internal, stationary magnet bar 20 is shown positioned proximal to rotating backing tube 14 and target material 10. A corresponding transverse cross sectional perspective view is shown in FIG. 2B that is taken along line IIB-IIB of FIG. 1A. The internal magnet bar 20 has a magnetic shunt 26 and magnets 27 and 28. The arrows depict the magnetic polarity according to common convention with the arrowhead pointing towards a north pole. The configuration of magnets 27 and 28 and shunt 26 result in magnetic field lines 30 that arch over and through target material 10. The apex of the field lines arching over and through the target is plotted as field line 29. Since, as is known, electrons tend to have concentrated density at the center of the arch, the principal erosion region of cylinder section 13 coincides with along field line 29 when racetrack 2 is generating plasma. This plasma being concentrated in the turnaround portion 3 relative to straight portion 4. In this case, field line 29 is roughly normal to the surface of the cylinder 1. The erosion zone at the at the cylinder portion 13 then is continuously focused over the same linear location at point 7 on the target tube and excessive wear occurs as shown by the worn target profile of cylinder section 13. As shown, the cylinder section 13 is worn to the backing tube 14 at point 7 while substantial target material remains unused along straightaway cylinder section 12, underlying straight portion 4.
Prior art attempts have been made to improve target utilization have met with limited success. One such prior art configuration is depicted in FIG. 3 and teaches away from the present invention. Like numerals used in FIG. 3 have the meaning ascribed thereto with respect to the preceding figures. FIG. 3 shows a cross-sectional view of U.S. Pat. No. 5,364,518. In this patent, the problem of poor target utilization of rotary magnetrons is recognized and the attempts to improve target utilization. FIG. 3 is based on FIG. 7B of U.S. Pat. No. 5,364,518 and the resulting target erosion profile is shown overlaid on this drawing, where a magnetic shunt 120 is added to the side of magnet 107 and shunt 106 to pull magnetic flux toward the end 125 of target tube cylinder 1. This is taught in U.S. Pat. No. 5,364,518 to widen the target erosion region at the turnaround region 13 and improves target utilization. An analysis of the proposed solution shows the apex of the resulting magnetic field lines 130 plotted as line 109. As shown, line 109 is off normal line 131 by angle θ, also referenced as 110. This geometry results in the erosion zone 114 starting out closer to the end of target cylinder 1. As the target cylinder 1 is eroded, the erosion zone follows line 109 and moves away from the end 125 of the target cylinder 1. Unfortunately, this has only a minimal benefit to overall target utilization. By moving the erosion zone 114 progressively toward the straight away section of cylinder 1 and underlying straight portion 4 of racetrack 2 with continued removal of target material 10, the erosion zone moves toward a high erosion region of the target and merely broadens the width of the erosion zone relative to that of FIGS. 1B and 2A.
This broadening is understood with reference to the following equation that approximates the terminal target cross section, t(l) when no further target sputtering can occur without risk of backing tube sputtering:t(l)=Dr(1−ek(l−l)f)2  (I)where Df is the final erosion depth and roughly models the width of the erosion zone with a smaller value of Df corresponding to a wider erosion zone, l is the lateral position and lf is the maximal erosion point denoted at 7 in the aforementioned drawings, and k is a fitting constant.
The approximate fit of equation (I) onto a conventional erosion profile of FIG. 2A is shown graphically as a dashed line in FIG. 2C. For a noimalized erosion profile where the initial target thickness is a unitless value of 1 and point 7 is at l=1, the depicted fit corresponds to two parameter fit for Df=0.48 and lf=0.88, where k=1. It is appreciated that the erosion profile of FIG. 3 is similarly fit with this expression with a best two parameter for Df=0.40 and lf=0.96, where k=1.
Thus, there exists a need for a magnet bar and an apparatus including the same that provides more efficient target utilization for rotary magnetrons. There further exists a need for moving the erosion zone away from the straightaway region of a proximal racetrack to afford an improvement in target utilization.